Air in-leakage in the back pass of boilers has significant negative impact on precipitator performance and fan horsepower as well as boiler efficiency and capacities. That same leakage can create cool ductwork internal surfaces, which lead to acid dewpoint corrosion by moisture condensation which causes severe duct deterioration, air heater corrosion, air heater plugging and fan corrosion. As with all air in-leakage, additional mass flow negatively impacts all fan and electrostatic precipitator capacity oriented components. An effective strategy in reducing overall air in-leakage has been developed and used in the southeast. This strategy is a combination of technologies and products applied in various back pass airflow applications with similar results. This paper will discuss case studies illustrating the strategy and results.

L.V. Sutton Unit No. 2 is a Babcock & Wilcox reheat type steam generator owned by Carolina Power & Light Company. The boiler which was built in 1954 was designed to provide a maximum continuous steam capacity of 775,000 lb/hr with a superheater outlet pressure of 1585 psig and temperature of 1005°F to a 100 Mw reheat - regenerative cycle turbine generator.

Recognizing the age of the ductwork from the air heater inlet through the stationary dampers at the I.D. fan outlets, along with reduced performance, Carolina Power & Light Company realized the need for significant rehabilitation. Air Flow Technologies Inc., a regular supplier, was engaged to accurately assess existing conditions, survey and recommend repair and replacement options.

To verify these recommendations, Mr. R. M. Shreiner, PE, was brought in by Air Flow Technologies to inspect, test, evaluate data, make recommendations, and predict performance improvements.

The basis for the evaluation by Air Flow Technologies was a static pressure check and oxygen monitoring, performed by plant personnel to determine system air leakage at various locations.

The following table represents this air in-leakage percent, as determined by measuring free oxygen in the flue gas system. The total average air in-leakage for both sides (fans) was 60%. Considering the age of the system, reasonable leakage should be no greater than 20% to 25%. Excessive air in-leakage will decrease the effective capacity of the ID fans and increase pressure drops downstream of the leakage, thus decreasing steam flow and MW outputs because of the extra capacity and static pressure required by the ID fans.

The following table represents Air In-Leakage Percentages at key locations:
 

Air In-Leakage	"A" Side	"B" Side
	Leakage %	Leakage %
Boiler Output Air Leakage	OK	OK
Boiler Output to Economy Outlet	2.31	5.91
Economy Outlet to ESP Inlet	4.15	2.06
Across ESP	1.31	1.47
ESP Outlet to A.H. Inlet	0.29	2.37
Across Air Heater	8.44	19.77
A.H. Outlet to ID Fan Inlet	39.6	33.3
Total In-Leakage	56.1	64.88

 

 

 

Average Total In-Leakage Both Sides 60.49%

The above data shows that nearly 60% additional air is being moved through the system resulting from air in-leakage. The largest contributor to this leakage is across the air heater and in the ductwork from the Air Heater Outlet to the ID Fans. Within the ductwork from the Air Heater to the ID Fans was a Mechanical Dust Collector which was used prior to the installation of Precipitators in 1974. The ductwork geometry, as well as the somewhat restricted flow through the mechanical dust collector, created additional static pressure losses which further reduced capacities.

After evaluation of the data and equipment, Air Flow Technologies recommended the replacement of the existing ductwork with a redesigned configuration that allowed for the elimination of the mechanical collector. New high temperature nonmetallic expansion joints were recommended for the new duct as well as at other locations where the existing metal joints were deteriorating. These recommendations were incorporated into the scope of the project.

Predicted Performance

It was predicted that the removal of the mechanical collector, adjusting the Air Heater seals, along with the installation of redesigned ductwork and new expansion joints would have the capability of increasing megawatt output by 13%. The system may not be operated at that higher rate, but the fans would provide this capability. The predicted increase in megawatt capacity was 4.5%. The improvement in ID fan capacity was predicted to be 36%.

Direct Cost of Amp Reduction =
Kilowatt * annual operating hours * cost per kWh

= 546.1 kW * 8000 hours/year * 0.02 $/kWh
= $ 87.383

Operating at peak load for approximately 500 hours produces the following:
= 2.0 Mw * 20 $/Mw * 500 hours
= $ 20,000.00

Precipitator Air In-leakage

Mayo Unit #1 is comprised of two Foster Wheeler pulverized coal fired boilers owned by Carolina Power & Light Company. Each steam generator is designed to provide 2,750,000 pounds per hour of steam at 1005° F and 2,580 psig at the superheater outlet to a single 817,600 KVA tandem-compound, quadruple flow, single reheat turbine.

Mayo Unit #1 was experiencing performance related problems associated with air in-leakage at the seal separating the precipitator hot roof and the penthouse. Poor performance of this seal allowed for leakage at several locations. The relatively cold ambient air quenched the hot roof causing localized buckling which further increased the leakage. The buckling also caused plate and electrode alignment problems. Air in-leakage into the precipitator caused severe performance challenges from ash re-entrainment, increased velocities, and allowing moisture to enter into the gas stream which caused severe opacity problems, induced spark over the insulators and warp collector plates. Due to these operating problems, the unit was not available for full load at various times.

Rather than making repairs similar in design to the original construction, Carolina Power & Light opted to install an ISOMEMBRANE® seal around the hot roof perimeter of the units four precipitators.

ISOMEMBRANE® is a product designed and applied by High Temperature Technologies, Inc., who is the North American Licensee for the product and a regular supplier to Carolina Power & Light Company. The ISOMEMBRANE® seal provided the ability to maintain an air tight seal over the existing expansion joint without having to remove or demolish the existing equipment. The ability to install over the existing seal allowed for very minor disruption to other activities scheduled on all four precipitators during this same outage. The challenges for the seal were that it had to handle 8 inches of perimeter expansion as well as be able to withstand the elements of weather as the seal was to be placed outside the unit.

In addition to the ISOMEMBRANE® seals, sixty four expansion joints were either replaced or were repaired during the outage. It should be noted that although the seventy two (72) expansion joints, repaired or replaced, all contributed to some extent in the total air in-leakage; the availability of the unit was affected primarily by the problems associated with the precipitator seals.

Actual Performance

The actual increase in MW capacity, as operated, was reported to be in excess of 4.5% during hot weather conditions when the boiler was previously ID fan limited. The ID fans are currently operated at 55% damper position compared to 100% prior to redesign. The fan motor amperage was reduced from an average of 193.7 to an average of 155.8 amps. The ID fan inlet pressure was reduced from an average of 18.5 in. to 13.5 in.

Much of the leakage in the duct, expansion joints, and fan casing was the result of corrosion caused while the unit was off-line by moisture and subsequent acid dewpoint attack of the duct surfaces. This was from the air that was pulled through the F.D. fans, air heater, ID fans and ductwork by the stack draft. It was decided to install guillotine dampers above the ID fan outlet in order to eliminate this stack draft and improve the long-term maintenance of the unit during periods of shutdown.

Poor system reliability as a result of the overall condition of the ductwork, expansion joints, and fan casing was the primary reason for the refurbishment project. Design changes in the ductwork configuration, as well as removal of the mechanical collector, resulted in additional operating savings. A summary of the project scope and costs are shown below.

°Project involved 100 tons of ductwork, 28 expansion joints, six dampers and new casing for both ID fans. All existing equipment was demolished and reinstalled during an 8 week turbine outage in 1995. The total cost including installation was $800,000.
°The potential savings from reduced fan amperage for the ID fan motors is $87,383.
L.V. Sutton Unit #2 was also able to produce an additional 2.0 Mw of electricity at peak operation. This would allow an additional $20,000
°Total potential savings exceed $100,000 per year.

In order to calculate a dollar value for the savings, the following calculations are used:

kW = 1.732 * Amps * Volts
Amps = 2 Motors * (193.7 - 155.8) = 75.8 Amps
Volts = 4160 Buss Volts

8000 hour/year of operation kW
= 1.732 * 75.8 amps * 4160/1000 kV
$0.02 / kWh kW = 546.1

°Prior to the outage, the total air in-leakage from the economizer outlet through the precipitator outlet was an average of 26%.

°After the outage, the total air in-leakage through the same location was 4%.

°The project involved the application of ISOMEMBRANE® seals on four (4) precipitators, replacing sixty four (64) expansion joints, and repairing eight (8) economizer outlet expansion joints using ISOMEMBRANE®.

The increased reliability and availability of the unit relating to the installation of the ISOMEMBRANE® at the penthouse seals was clearly recognized by plant operations. The costs and savings associated with the improved reliability of the precipitator has been significant.

In conclusion, operational performance can be dramatically increased through economically efficient processes designed to abate air in-leakage. Precipitator performance will improve through reduction of opacity and lower gas velocities. Air in-leakage also contributes heavily to limiting units to ID fan capacity. By reducing air in-leakage, we have shown a tremendous increase in unit efficiency and performance.

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